Changes

==Braided-channel deposits==

==Braided-channel deposits==

Braided channels are marked by successive divisions and rejoinings of the flow around [[alluvial]] islands. Most braided river channels are characterized by a dominant bedload transport of sediment, high variations in water discharge, high downstream gradients, and large width-depth ratio of channels. Most braided rivers display rapid and continuous shifting of sediment and position of channels. These channels are found in all climate zones, but, because of their dependence on erratic discharge and high bedload, they are most common in arid and arctic settings. Braiding characteristics of the channel often extend all the way into the delta plain. However, one of the largest braided channels in the world, the Brahmaputra River, has formed in a humid climatic setting. Lateral migration of c annels can be dramatic, as in the Brahmaputra River, where lateral migration rates of several thousand meters during a single flood are not uncommon.<ref name=Coleman_1969>Coleman, J. M., 1969, Brahmaputra River: channel processes and sedimentation: Sedimentary Geology, v. 3, p. 129-239.</ref> In the Rosi River (a tributary of the Ganges River), lateral migration of the channel over the past two centuries has been about 170 km; in a single year the channel may shift over 30 km laterally.<ref name=Reineckandsingh_1973>Reineck, H. E., and I. B. Singh, 1967, Primary sedimentary structures in the Recent sediments of the Jade, North Sea: Marine Geol., v. 5, p. 227-235.</ref> Because of high lateral migration rates and shallow depths of scour, most braided-channel deposits display high lateral continuity but are rather thin (rarely over 30 m thick).

Braided channels are marked by successive divisions and rejoinings of the flow around [[alluvial]] islands. Most braided river channels are characterized by a dominant bedload transport of sediment, high variations in water discharge, high downstream gradients, and large width-depth ratio of channels. Most braided rivers display rapid and continuous shifting of sediment and position of channels. These channels are found in all climate zones, but, because of their dependence on erratic discharge and high bedload, they are most common in arid and arctic settings. Braiding characteristics of the channel often extend all the way into the delta plain. However, one of the largest braided channels in the world, the Brahmaputra River, has formed in a humid climatic setting. [[Lateral]] migration of channels can be dramatic, as in the Brahmaputra River, where lateral migration rates of several thousand meters during a single flood are not uncommon.<ref name=Coleman_1969>Coleman, J. M., 1969, Brahmaputra River: channel processes and sedimentation: Sedimentary Geology, v. 3, p. 129-239.</ref> In the Rosi River (a tributary of the Ganges River), lateral migration of the channel over the past two centuries has been about 170 km; in a single year the channel may shift over 30 km laterally.<ref name=Reineckandsingh_1973>Reineck, H. E., and I. B. Singh, 1967, Primary sedimentary structures in the Recent sediments of the Jade, North Sea: Marine Geol., v. 5, p. 227-235.</ref> Because of high lateral migration rates and shallow depths of scour, most braided-channel deposits display high lateral continuity but are rather thin (rarely over 30 m thick).

[[file:M31F2.jpg|thumb|200px|{{figure number|1}}Photographs of bedding in a braided channel deposit. A. Large-scale cross-bedding in the lower part of a fining-upward cycle on a braided channel. B. Trough-shaped cross-bedding in lenticular sets that form the overlying zone in a fining-upward cycle of a braided channel. C. Ripple drift bedding separated by parallel sand laminations.<ref name=Colemanandprior_1981>Coleman, J. M., and D. B. Prior, 1981, [http://archives.datapages.com/data/specpubs/sandsto2/data/a058/a058/0001/0100/0139.htm Deltaic environments of deposition], in P. A. Scholle and D. Spearing, eds., Sandstone depositional environments: [http://store.aapg.org/detail.aspx?id=627 AAPG Memoir 31], p. 139-178.</ref>]]

[[file:M31F2.jpg|thumb|200px|{{figure number|1}}Photographs of bedding in a braided channel deposit. A. Large-scale [[cross-bedding]] in the lower part of a fining-upward cycle on a braided channel. B. Trough-shaped cross-bedding in lenticular sets that form the overlying zone in a fining-upward cycle of a braided channel. C. Ripple drift bedding separated by parallel sand laminations.<ref name=Colemanandprior_1981>Coleman, J. M., and D. B. Prior, 1981, [http://archives.datapages.com/data/specpubs/sandsto2/data/a058/a058/0001/0100/0139.htm Deltaic environments of deposition], in P. A. Scholle and D. Spearing, eds., Sandstone depositional environments: [http://store.aapg.org/detail.aspx?id=627 AAPG Memoir 31], p. 139-178.</ref>]]

Individual channels split around numerous mid-channel islands or braid bars. During floods erosion occurs on the upstream ends and lateral sides of bars and eroded material is added to the downstream side of the bar. Because each channel has a different depth, lateral migration of the channel results in scour to differing depths. Multiple fining-upward cycles (which are commonly truncated) occur within the resulting sand body. Each fining-upward cycle (owing to deposition of a laterally migrating channel) is characterized by a scoured base and overlying sets of large-scale cross-bedding in which individual sets are up to 1 m thick ([[:file:M31F2.jpg|Figure1A]]). This lower sequence of large-scale cross-bedding can attain thicknesses of up to 7 m. Small-scale ripple bedding, scour and fill structures, organic trash, and clay layers are occasionally found.

Individual channels split around numerous mid-channel islands or braid bars. During floods erosion occurs on the upstream ends and lateral sides of bars and eroded material is added to the downstream side of the bar. Because each channel has a different depth, lateral migration of the channel results in scour to differing depths. Multiple fining-upward cycles (which are commonly truncated) occur within the resulting sand body. Each fining-upward cycle (owing to deposition of a laterally migrating channel) is characterized by a scoured base and overlying sets of large-scale cross-bedding in which individual sets are up to 1 m thick ([[:file:M31F2.jpg|Figure1A]]). This lower sequence of large-scale cross-bedding can attain thicknesses of up to 7 m. Small-scale ripple bedding, scour and fill structures, organic trash, and clay layers are occasionally found.

Overlying this unit is a zone displaying finer grain size and composed of lenticular-shaped units of large-scale cross-bedding (trough type) intercalated with zones of climbing ripple laminations, horizontal laminations, and ripple cross-bedding ([[:file:M31F2.jpg|Figure 1B]]). In some places small-scale laminations are present within certain zones. The uppermost unit of the fining-upward cycle displays horizontal laminations separating well-defined, near-horizontal sets of steeply dipping ripple-drift bedding ([[:file:M31F2.jpg|Figure 1C]]). Small-scale convolute laminations and burrowed sand and silt layers are common in the uppermost part of the unit. However, scouring by later migration of the channel often removes this upper burrowed section.

Overlying this unit is a zone displaying finer [[grain size]] and composed of lenticular-shaped units of large-scale cross-bedding (trough type) intercalated with zones of climbing ripple laminations, horizontal laminations, and ripple cross-bedding ([[:file:M31F2.jpg|Figure 1B]]). In some places small-scale laminations are present within certain zones. The uppermost unit of the fining-upward cycle displays horizontal laminations separating well-defined, near-horizontal sets of steeply dipping ripple-drift bedding ([[:file:M31F2.jpg|Figure 1C]]). Small-scale convolute laminations and burrowed sand and silt layers are common in the uppermost part of the unit. However, scouring by later migration of the channel often removes this upper burrowed section.

[[file:M31F3.jpg|thumb|300px|{{figure number|2}}Summary diagrams illustrating the major characteristics of braided channel deposits (letters on the vertical section refer to core or outcrop photographs).<ref name=Colemanandprior_1981 />]]

[[file:M31F3.jpg|thumb|300px|{{figure number|2}}Summary diagrams illustrating the major characteristics of braided channel deposits (letters on the vertical section refer to core or outcrop photographs).<ref name=Colemanandprior_1981 />]]

[[:file:M31F3.jpg|Figure 2]] summarizes the major characteristics of braided-channel deposits, including lateral relationships (block diagram in upper left); typical vertical sequence (upper right), including grain size, directional properties, dip angles, relative porosity, and sedimentary structures; sand body isopach map (lower left); and representative electric logs (lower right) at selected sites to show variation in log shape. As this diagram illustrates, the typical vertical sequence is characterized by multiple stacked fining-upward cycles of deposition (each representing deposition by a migratory channel). Directional properties within each cycle often display narrow directional spread and are fairly representative of the long-axis orientation or downstream direction of the channel. High dip angles are most often associated with large-scale cross-bedding and distorted layers.

[[:file:M31F3.jpg|Figure 2]] summarizes the major characteristics of braided-channel deposits, including lateral relationships (block diagram in upper left); typical vertical sequence (upper right), including grain size, directional properties, [[dip]] angles, relative porosity, and sedimentary structures; sand body isopach map (lower left); and representative electric logs (lower right) at selected sites to show variation in log shape. As this diagram illustrates, the typical vertical sequence is characterized by multiple stacked fining-upward cycles of deposition (each representing deposition by a migratory channel). Directional properties within each cycle often display narrow directional spread and are fairly representative of the long-axis orientation or downstream direction of the channel. High dip angles are most often associated with large-scale cross-bedding and distorted layers.

The isopach map shows a laterally continuous sand body, often extending 20-50 km laterally in a direction perpendicular to the downslope channel direction. Most braided channels display rather uniform thicknesses across the entire sand body (averaging 15 to 25 m thick) and localized deeper sand-filled scoured pods. Log response often shows an overall blocky shape, with numerous sharp "kickouts" representing local coarse-sand-filled scours. Locally numerous fining-upward cycles can be well defined on the logs. Although distinctive cycles can often be discerned from log data, it is very likely that individual units cannot be carried laterally any great distance, and presence of the numerous thin silt and clay layers discourages thinking that reservoir continuity may extend for great dis ances. Exposures in some of the tar sands in Canada show a lack of reservoir continuity as tars are concentrated along distinct layers within the overall sand body. Potential for porosity traps is great in the braided-channel environment.

The isopach map shows a laterally continuous sand body, often extending 20-50 km laterally in a direction perpendicular to the downslope channel direction. Most braided channels display rather uniform thicknesses across the entire sand body (averaging 15 to 25 m thick) and localized deeper sand-filled scoured pods. Log response often shows an overall blocky shape, with numerous sharp "kickouts" representing local coarse-sand-filled scours. Locally numerous fining-upward cycles can be well defined on the logs. Although distinctive cycles can often be discerned from log data, it is very likely that individual units cannot be carried laterally any great distance, and presence of the numerous thin silt and clay layers discourages thinking that reservoir continuity may extend for great dis ances. Exposures in some of the tar sands in Canada show a lack of reservoir continuity as tars are concentrated along distinct layers within the overall sand body. Potential for porosity traps is great in the braided-channel environment.

The channel abandoned when a river switches its course to another site is most commonly filled with silty clays, organic clays, and organic trash. The clay-filled channel, therefore, results in numerous nearly isolated sand bodies in the overall meander belt providing innumerable stratigraphic trapping possibilities. This is illustrated in the upper left diagram of Figure 4. The vertical sequence in the meander point-bar sand body is illustrated in the upper right diagram of [[:file:M31F4.jpg|Figure 3]].

The channel abandoned when a river switches its course to another site is most commonly filled with silty clays, organic clays, and organic trash. The clay-filled channel, therefore, results in numerous nearly isolated sand bodies in the overall meander belt providing innumerable stratigraphic trapping possibilities. This is illustrated in the upper left diagram of Figure 4. The vertical sequence in the meander point-bar sand body is illustrated in the upper right diagram of [[:file:M31F4.jpg|Figure 3]].

Most commonly, the vertical sequence shows a fining-upward grain size relationship; a few coarser layers are found near the upper one-third of the sand body. The sand body has a scoured base, and often coarse, organic trash (logs, limbs and clay clasts) is found intercalated with the sandy units. Thin clay and silt layers often separate coarse sandy units. Above this basal unit is normally a massive, thick sand unit displaying large-scale cross-bedding with occasional contorted layers and thin laminations of organic debris. The large-scale bedforms migrate primarily during periods of high flood.

Most commonly, the vertical sequence shows a fining-upward grain size relationship; a few coarser layers are found near the upper one-third of the sand body. The sand body has a scoured base, and often coarse, organic trash (logs, limbs and clay clasts) is found intercalated with the sandy units. Thin clay and silt layers often separate coarse sandy units. Above this basal unit is normally a massive, thick sand unit displaying large-scale [[cross-bedding]] with occasional contorted layers and thin laminations of organic debris. The large-scale bedforms migrate primarily during periods of high flood.

[[file:M31F5.jpg|thumb|300px|{{figure number|4}}Photographs of bedding in a meander point bar. A. Cyclic flood deposits in a point bar. B. Small-scale cross-stratification and organic debris. C. Climbing ripple sequence capped by convolute laminations. D. Highly contorted bedding in point-bar deposits. E. Soil zones alternating with ripple laminations in upper part of point-bar deposits.<ref name=Colemanandprior_1981 />]]

[[file:M31F5.jpg|thumb|300px|{{figure number|4}}Photographs of bedding in a meander point bar. A. Cyclic flood deposits in a point bar. B. Small-scale cross-stratification and organic debris. C. Climbing ripple sequence capped by convolute laminations. D. Highly contorted bedding in point-bar deposits. E. Soil zones alternating with ripple laminations in upper part of point-bar deposits.<ref name=Colemanandprior_1981 />]]